Recently, unmanned aerial vehicle (UAV) communication has received considerable research interests due to its flexible deployment and the dominance of line-of-sight links [1]}, [2]}. UAV can be deployed as a relay when there is no direct link between any two nodes. Specifically, Zeng et al. [3]} first studied the trajectory and power allocation for UAV-relay systems. In [4]}, the reliability of the UAV relay was analyzed in terms of outage probability and bit error rate.
